Asphalt concrete, also known as asphalt pavement, is a composite material comprising mineral aggregate and asphalt (bitumen) binder which hardens to form a robust surface. Early in its life, the asphalt concrete has the ability to resist erosion, compressive deformation, and load fracture. However, as the asphalt concrete ages, the asphalt oxidizes, decreasing the binding capacity, increasing the viscosity, and further hardening the asphalt concrete. This leads to cracks in the asphalt concrete surface. Small cracks in the surface of the asphalt concrete allow for further water intrusion into the asphalt concrete, and the oxidized asphalt concrete is more susceptible to structural weakening and failure due to the reduction in binding capability.
Several surface treatment methods for aged asphalt are known which comprise adding a thin layer of modified asphalt concrete to extend the structural life of the asphalt concrete. Such surface treatment methods include chip seal, slurry seal, and microsurfacing treatments. However, the known slurry seal and microsurfacing treatments do not themselves form a water-tight layer. Without a water-tight layer, the older, underlying asphalt concrete continues to oxidize and crack thereby causing reflective cracks in the new surface layer. Furthermore, the slurry seal or microsurfacing layer does not adhere well to a highly oxidized asphalt concrete surface.
To address this problem and provide a water barrier between the old asphalt concrete and the new surface layer, a tack coat can be applied to the old asphalt concrete prior to slurry seal or microsurfacing operations. The tack coat typically comprises a cationic or anionic diluted asphalt emulsion comprising asphalt, water, latex polymer, emulsifier, and an optional aromatic recycling agent. The tack coat is applied to the old asphalt concrete surface and cured to form a continuous film on the older asphalt concrete which is air and water-tight. The film prevents further oxidation and water damage of the old asphalt concrete. If the tack coat includes a large amount of polymer latex, it can also reduce the penetration of cracks into the new surface layer (reflective cracks) and improve the adhesion between the old asphalt concrete and the new layer. The optional aromatic recycling agent can rejuvenate the older, oxidized asphalt concrete by restoring the aromatic content of the oxidized asphalt.
Another way to prevent reflective crack formation in the new surface layer is to apply a chip seal to the older asphalt pavement prior to slurry seal or microsurfacing applications. The process of applying a chip seal prior to slurry seal or microsurfacing is also known as cape seal, as it was first developed in Cape Town, South Africa. During the chip seal process, the asphalt pavement surface is first sprayed with an asphalt emulsion comprising asphalt, water, an emulsifier (typically a cationic rapid set emulsifier), and optionally a polymer latex. Then, aggregate is spread onto the asphalt emulsion and the aggregate is compacted with pneumatic tires. The chip seal surface then must be cured for 5-10 hours. Before the surface can be opened to regular use, the chip seal surface must also be swept to remove loose aggregate, typically 3-5 hours after application of the asphalt emulsion and aggregate. Chip seal uses single sized aggregate which forms a surface with a plurality of air voids. The air voids act as termination points for cracks from the older asphalt pavement thus preventing reflective crack formation in the new surface layer. After application and curing of the chip seal layer, the slurry seal or microsurfacing layer is applied. The slurry seal or microsurfacing layer then provides a traction surface.
Both the tack coat layer and the chip seal layer described above must be cured with sufficient strength to support traffic before the final surface treatment is applied. As a result, it takes a minimum of three hours for the tack coat layer or chip seal layer to be sufficiently cured such that the microsurfacing or slurry seal layer can be applied and ultimately before the asphalt concrete surface can be subjected to normal use. Moreover, with the above-described processes known in the art, two separate operations are required before an aged asphalt concrete surface is sufficiently restored and ready for normal use. This results in prolonged lane or road closure, which is undesirable. Furthermore, the need for two separate application steps with a curing step in between adds to the cost of the treatment due to the cost of using two road crews and their equipment, in addition to the costs related to prolonged lane closure.